1. Field of the Invention.
The present invention relates to a diverless process for tapping a pressurized subsea pipeline without removing the pipeline from service. This diverless process utilizes specialty clamping equipment, tapping equipment and remote operating vehicles for operating at water depths beyond that accessible by divers.
2. Prior Art
Various kinds of underwater pipeline work have been carried out using divers equipped with various tools. Deep water conditions entail high subsea pressure and dangerous conditions. Manned or diver equipped operations are not suitable if the pipeline is at a depth which is inaccessible by divers.
Remote operating vehicles perform visual and operational functions that are more limited than those functions performed by divers. To compensate for this limitation, processes must be developed to adapt the available technology to the work requirements. These processes are typically referred to as "diverless". Developed diverless processes are easily adapted for use using divers and/or control from the sea surface in diver accessible water depths.
It has been proposed in the past to utilize a diverless underwater pipe tap system. De Sivry et al. (U.S. Pat. No. 4,443,129) discloses an example of a diverless underwater pipe tap system including a lifting device to engage and displace the pipe, a working platform installed vertically above the tapping region and various tools used to carry out operations including concrete stripping, ovality or roundness measurement, and a tapping operation. De Sivry et al. contemplates cutting out a section of the pipeline. There is no disclosure or suggestion of a tap procedure while the pipeline is under pressure and de Sivry would not be suitable for such an operation.
Likewise, various hot tap machines for tapping into a pipeline while the pipeline is pressurized have been disclosed. Reneau et al. (U.S. Pat. No. 4,223,925) discloses one example of a hot tap machine for an underwater pipeline. It is advantageous to perform work on the pipeline without interrupting the flow of liquids or gases therethrough.
It is also known to provide a remote operated subsea vehicle (ROV) controlled from the surface which includes thrusters or other mechanisms for propelling it through the water in response to a source of power at or controlled from the surface.
Brooks et al. (Re27,745) illustrates an example of a submersible wireline robot unit that may be controlled remotely.
None of the foregoing would be suitable for deep water conditions where the external subsea pressure is extreme.
Given that the life spans of certain deep water oil and gas developments will be coming to an end in the near future, there will be an increasing number of existing pipelines which have excess transport capacity, thereby leaving excess pipeline infrastructure in deep water locations.
If production from new fields can be tied into existing pipelines, then the investment in the pipeline can be spread out over more than one development. A diverless hot tap capability would allow the extension of pipelines from new fields to be connected to existing pipelines. This will reduce the overall length of the pipeline required to support a new field.
It would also be possible to lay new trunk lines into areas in order to support multiple future field developments which would use lateral pipelines without having to pre-install connection facilities and valves at predetermined locations along the pipeline.
Additionally, in deep water oil and gas developments, the cost of the pipeline infrastructure is estimated to be the majority of the cost of all of the installed subsea equipment.
As an alternative to diverless hot tap systems, it would be possible to take an existing line out of service, cut a section of the pipeline and bring it to the surface, install a valve, lower the section back to the bottom and reconnect with a suitable jumper. Such an approach is not only time-consuming but requires the pipeline to be out of service. It would also be possible to pre-install blind branch connection facilities in the pipeline for future tie-ins. A disadvantage is that a great number of branch facilities would have to be installed to cover all possible future tie-in locations, a procedure that would be both time-consuming and expensive.
Notwithstanding the foregoing, there remains a need for a diverless process of tapping a deep water subsea pipeline under high pressure without removing the pipeline from service.